The present invention relates to automatic transmissions having a layshaft type of kinematic arrangement, and particularly to such automatic transaxles having dual input shafts.
Automatic transmissions in vehicles, which transmit power between an input and an output, either over a continuously variable range of speed ratios or in discreet step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, hydraulic friction clutches and breaks, hydraulic pump, and gear meshing. Moreover, these conventional automatic transmissions tend to be higher cost than is desirable.
To reduce costs and improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce these parasitic losses except gear mesh losses. It is basically a transmission with a kinematic arrangement similar to a conventional manual transmission, but with gear changes actuated by an automated system rather than by driver intervention. An ASM transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next gear ratio, and then restoring torque at the input. A significant drawback of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change.
Another type of automatic transmission is a dual clutch layshaft transmission. Dual clutch layshaft transmissions are essentially two ASM transmissions combined into one, with a first input shaft from a first clutch providing power input to odd-numbered forward gears and a second input shaft from a second clutch providing power input to even-numbered forward gears. While operating in an odd-numbered gear, transmission components can be prepared for the next even-numbered gear. Consequently, shifts between odd-numbered and even-numbered forward gears can be accomplished with little or no power flow interruption.
The reverse function for these dual clutch layshaft transmissions is typically included in this kinematic arrangement by using sliding gear kinematics with spur gears that come into and out of mesh when shifting into and out of reverse, by extending the length of the input shafts and including the gears on this extended portion, or by employing two output shafts that have separate pinion gears mating with the ring gear of the differential. However, each of these arrangements has undesirable characteristics.
The spur gears employed with a sliding gear arrangement create undesirable noise problems and thus are not the most desirable of kinematic arrangements. Moreover, a mechanism is required to move the reverse gear into and out of engagement. In order to avoid these problems, it is preferable that constant mesh gears be employed for the entire kinematic arrangement.
Extending the length of one or both of the input shafts to accommodate a reverse gear set allows for the use of constant mesh gears, thus eliminating the noise problem. However, this may create problems packaging the transaxle in the engine compartment of the vehicle. Motor vehicles in which the front wheels are the driven wheels and the engine and transmission are located in a forward engine compartment generally require the engine and transmission to be arranged in a space whose lateral dimension is limited by the spacing between the front wheels. The engine compartments of such vehicles are both narrow and short, which creates an acute need to minimize the package space occupied by the transaxle, particularly in its lateral dimension. Thus, an arrangement with an extended input shaft to accommodate a reverse gear may cause interference between the transaxle and a component in the engine compartment, such as a frame rail of the vehicle.
The other option, which employs two output shafts with separate pinion gears, also has its drawbacks. The cost of the additional output shaft, extra pinion gear, and extra bearings and support structure to support the shaft can increase the cost of the transaxle more than is desired, as well as create other types of packaging concerns in the engine compartment.
Thus, it is desirable to have a kinematic arrangement for an automated manual type of transaxle having two input shafts where a constant mesh reverse gear arrangement is provided for while still minimizing the length of the input shafts.